Source: HeatingAndVentilating.net - http://www.heatingandventilating.net/
Ian Boyd, a dedicated VRF engineer for LG Air Conditioning, looks at some of the technical points on the overall performance of a heat recovery VRF system when the cooling needs to switch to heating
IT is widely acknowledged that heat recovery variable refrigerant flow (VRF) systems aren't always at their most efficient when changing from one mode to another - something that can happen several times a day in our indecisive climate.
This drop in performance is because of a basic principle: if an indoor unit is in heating mode and the user wants cooling (or vice-versa), the indoor unit cannot just change mode instantly.
If this were to happen, it would cause a rush of refrigerant (because of the pressure differential between cooling and heating) which is noisy and would probably cause long-term vibration damage to the refrigerant piping.
Typically, in heating mode, the indoor unit will run at about 400psi, and in cooling it will be running at about 120psi - a big difference in pressure.
Pressure equalisation to accommodate this can be done a number of ways. Generally, it takes the form of the whole system slowing down to reduce pressures. Then the single unit changes over, and then the system loads up again. Just think of the energy consumption and loss of comfort.
The effect of doing it this way is that the capacity of all the indoor units connected to this system (regardless of which mode they are in) is affected when any one indoor unit changes mode.
It takes time and energy for the system to regain capacity. Then, if another indoor unit changes mode, the capacity is affected yet again, and rooms will become uncomfortable as the air conditioning or heating is not running at the correct capacity for potentially long periods of time.
So, what is the answer? There are products available that supersede and nullify this nuisance. They change the mode of an individual unit quickly with no effect on its neighbours. They have true automatic changeover.
When any indoor unit is changed from one mode to another, it has no capacity effect on the rest of the system, as the outdoor unit does not have to slow down to reduce the pressure for changeover. The indoor unit which requires the mode change is equalised in 90s or less (at the heat recovery box), and then changes mode. It then quickly regains capacity.
This eliminates capacity loss for all other indoor units connected to the system, because of the outdoor unit's ability to continue to run during mode change situations. This means comfort is not sacrificed anywhere else in the system for the sake of one indoor unit changing from one mode to another.
With heat recovery, it is possible to have simultaneous heating and cooling from just one VRF outdoor unit to ensure the best possible comfort conditions, regardless of user demands or the vagaries of space or seasonal conditions.
Specific VRF heat recovery systems allow indoor units to be designated or switched from heating to cooling and vice-versa. This gives complete flexibility to any site which has varying requirements - cellular offices, hotel rooms, gyms or restaurants for example.
For simultaneous heating and cooling, the heat absorbed from indoor units in cooling mode is recycled and reused for indoor units requiring heating, thus maximising power use of optimising comfort levels.
There is no capacity loss anywhere else in the system when an indoor unit changes mode from heating to cooling, or vice-versa. Also, as the system is non-stop during mode changeover, there is reduced energy consumption from stabilised interior conditions.
Further, this overcomes the main drawback of three-pipe heat recovery systems - namely the heat/cool changeover lag.
Units such as these are true state-of-the-art air conditioning systems maximising energy efficiency, while maintaining the ability to change from one mode to another quickly - with no capacity loss.
To give a real example: in a hotel, if one guest switches to cooling mode when all others are in heating mode, there will be no loss of performance to any of the rooms already in heating mode. This used to be a common problem with heat recovery systems.
This attribute applies whether the system has a few indoor units connected, or if it has many. Just one of these types of VRF heat recovery systems can handle up to 40 indoor units.
These VRF heat recovery systems are not major engineering feats of endurance - they are easy to install, as there are fewer branch joints, less piping, and therefore fewer brazing points. This all leads to reduced cost of installation materials, and therefore less installation time required on site.